Sets an offset that is added to the device coordinates determined
by the CTM when drawing to the surface. One use case for this function
is when we want to create a surface that redirects drawing
for a portion of an onscreen surface to an offscreen surface in a
way that is completely invisible to the user of the Cairo
API. Setting a transformation via ccontext:Translate() isn't
sufficient to do this, since functions like
ccontext:DeviceToUser() will expose the hidden offset.
Note that the offset affects drawing to the surface as well as
using the surface in a source pattern.